In telecommunication systems for speech transmission, the subscriber line between a telephone exchange and the subscriber's telephone set is supplied with power by amplifiers in a line circuit in the telephone exchange. The amplifiers, which must be able to transmit a speech signal, are complicated and develop relatively large idling power. In digital telephone communication systems the mentioned power supply is also active for carrying out functions in the subscriber's set even when the receiver is on its cradle. It has been proposed to equip the subscriber set with a local power supply for reducing the power development in the line circuit in the telephone exchange. This is expensive and complicated, however, and there is the risk of deteriorated function in the telecommunication system. In the U.S. Pat. No. 4,388,497 there is proposed an apparatus for reducing the power development in the line circuit. This apparatus is complicated, and the telephone set is supplied with power from two separate power sources which are respectively connected and disconnected when the subscriber lifts his receiver.